Optoelectronic modules can be configured to acquire 3D data of a scene using stereo-, structured light-, or other three-dimensional (3D) imaging technologies. These modules can include an illumination assembly for illuminating a scene with features (e.g., discrete features), features such as projected patterns or textures. The acquisition of accurate, precise 3D data depends, in part, on the proficiency of the illumination assembly to generate features with various characteristics. For example, the illumination assembly should generate features with high-intensity and high contrast. Moreover, the illumination assembly should generate features with customizable illumination characteristics over its full field-of-view (FOY); for example, in some cases features should be generated with homogenous intensity over its full FOV. Furthermore, the illumination assembly should generate features with consistent optical quality over a range of operating temperatures.
An illumination assembly configured to illuminate scenes with high-intensity, high-contrast features includes an optical assembly and an illumination source. Light originating from the illumination source is typically characterized by a numerical aperture; that is, light is emitted within an emission angle (i.e., opening angle) with respect to an emission axis. When an optical assembly is paired with an illumination source having a large numerical aperture, the optical assembly is often configured with an aperture stop to define a certain amount of vignetting in order to mitigate optical aberrations.
For example, an optical assembly can be configured with a single aperture stop, which in turn can result in a small exit pupil (i.e., an aperture stop with a small diameter). Single aperture stops/small exit pupils effectively mitigate detrimental effects (e.g., vignetting) by blocking stray light propagating through the optical assembly, stray light that would otherwise reduce the optical quality of the features. However, since an aperture stop also blocks some of light generated by a light source, the optical assembly configured with an aperture stop is inherently inefficient; that is, some light generated by the light source is not used to illuminate the scene. On the other hand, the design of optical assemblies configured to generate features suitable for the acquisition of 3D data that do not require the use of aperture stops is non-trivial. Such optical assemblies are particularly demanding to simulate, analyze and test.
Moreover, as the optical assembly includes one or more optical elements within the optical assembly, one or more of these optical elements can be susceptible to thermally induced optical instabilities (e.g., changes in refractive index). This is a particular concern for optical elements composed of plastics, where thermal instabilities can cause changes in both the physical dimensions and refractive index of the optical elements. Both of these concerns can be exacerbated by the choice of illumination source. For example, an optical assembly can include a laser/laser array, such as edge-emitting lasers or vertical-cavity surface-emitting lasers (VCSELs) as the illumination source. In some optical assemblies, lasers can effectively produce high-intensity, high-contrast features; however, the high output power of lasers can lead to thermally induced optical instabilities as described above.